Work continues in the character of mitochondrial mRNA's; we have now demonstrated beyond serious doubt that Ehrlich tumor cell mitochondria contain messages encoded in the nucleus which are translated by the mitochondria into polypeptides present in the mitochondria. Furthermore, careful 2-D analysis now shows that the total number of peptides synthesized in mt are greater than the mitochondrial genome unless there is symmetrical transcription or a great deal of overlap. Tumor cells treated with alkylating agent undergo striking changes in transcription patterns depending on the agent. These involve alterations of size distribution of newly synthesized nuclear RNA chains and polysomal mRNA's. At the same time there are changes in polysome content and stability and in mRNP's and in polyA content.